Esters of phthalic acid are used to a great extent as plasticizers, in particular for polyvinylchloride. The alcohol components customarily used are primary aliphatic alcohols having 8 to 10 carbon atoms, among which 2-ethylhexanol currently is of the greatest importance. Phthalic esters of alcohols having less than 8 carbon atoms in the molecule lead to plasticizers having good gelling power. However, their relatively high volatility is a disadvantage. Phthalic esters which are derived from primary aliphatic alcohols having more than 10 carbon atoms, in contrast, gel more slowly and are less cold-resistant.
The properties of the phthalic ester plasticizers are influenced by the branching of the carbon chain, in addition to the size of the alcohol molecules. Thus, alcohols with a low degree of branching give ester plasticizers of high cold flexibility. Substantially linear alcohols having 8 to 10 carbon atoms in the molecule are therefore gaining increasing importance as the alcohol component of phthalic esters. A precondition for their use is that they be available in large amounts and at a reasonable cost.
According to German Patent 2 855 421, phthalates of 9 carbon alcohols, useful as plasticizers, are obtained by hydroformylation of 8 carbon olefins, hydrogenation of the reaction product, and esterification of the resultant 9 carbon alcohols with phthalic anhydride. 3% to 20% by weight of the starting olefins have an isobutane skeleton in each molecule chain, less than 3% by weight of the olefins has a quaternary carbon, and more than 90% by weight of the total amount of olefins is present as n-octenes, monomethylheptenes, and dimethylhexenes. In addition, the weight ratio of the total amount of n-octenes and monomethylheptenes to the dimethylhexenes is more than 0.8.
Phthalic esters of 10 carbon alcohols are the subject matter of European Patent Application 03 66 089. The alcohols used for the esterification are in the form of a mixture produced by hydroformylation of a butene fraction, aldol condensation of the resulting aldehyde mixture, and subsequent hydrogenation. The hydroformylation step, according to the process description, is not subject to any restrictions. Thus, not only cobalt, but also rhodium, can be used as catalyst, and the addition of an organic compound of trivalent phosphorus is not excluded.
Another way to obtain didecyl phthalic mixtures is described in European Patent Application 04 24 767. The esters are prepared by a multi-stage process by dimerization of butene mixtures, hydroformylation, and hydrogenation of the resulting octene mixture to a nonanol mixture, dehydration of the nonanol mixture to form a nonene mixture, followed by hydroformylation and hydrogenation of the nonene mixture to form a decanol mixture.
The known processes do not satisfy all of the requirements, both economic and technical, which are made of a process to be carried out on an industrial scale. The starting materials are either not available in sufficient quantities, or they are not inexpensive, or the conversion of the starting materials into the desired alcohols is associated with excessively complex (and hence costly) processes. In multi-stage processes, which include the hydroformylation of butene, the n-valeraldehyde content, in particular, of the hydroformylation product should be as high as possible, to promote the formation of straight-chain alcohols or alcohols which are only slightly branched.